Stress corrosion and fatigue in structures causes crack growth. This is due to the metal slowly becoming brittle when there is a concentration of stress within a short distance of the crack tip. The crack then advances to a zone boundary in a series of discrete microfracture events, where the microfractures can take place either intergranularly or transgranularly. Tougher undamaged material at the zone boundary stops the crack advancing. The cycle of cracking is then repeated, starting again with a concentration of stress at or near the crack tip.
Under normal operating conditions, damage such as cracking in a structure develops slowly over time. However, if the structure is operating outside its normal range, a large amount of damage may occur within a short time. In addition, damage caused by stress to a structure is not limited to cracking and may also include fretting, pitting and rubbing. It is therefore essential that structures be monitored regularly so that damage may be detected and repaired, or further damage prevented if the damage is not advanced.
Cracking and fracturing is known to cause particular problems in aircraft, pressure vessels and oilrigs, as well as in large structures such as bridges. As cracking occurs, the cracks produce bursts of acoustic energy as wideband ultrasonic emissions in the structure where the cracking is taking place, known as acoustic emissions. The properties of the waveform of the acoustic emissions, such as □t values representing the differences between the times that bursts of acoustic energy are received at different locations, frequency, amplitude, rise time etc are dependent on the size of the crack and how rapidly it propagates through the structure. Therefore cracks can be identified by their acoustic emission signature, which can be detected using acoustic sensors as acoustic emission sensors.
US 2003/0140701, the disclosure of which is hereby incorporated by reference, discloses a method of detecting and monitoring damage in a structure by receiving electrical signals continuously over a period of time as pulses representing a burst of acoustic energy from a plurality of acoustic sensors carried by the structure. The bursts of acoustic energy represent emissions from sites of damage. The burst is processed to obtain a smoothed envelope waveform. Wave-shape information and time information is determined and stored for each burst. If a burst is detected at three or more sensors, the difference in the time of arrival of the bursts at the sensors is determined as □t values. The □t values are then used to accumulate the bursts to determine if a threshold for the bursts is exceeded. If so, the burst data is stored to represent structural damage together with non-acoustic parameters.
However, there is a limitation in this system. When the health of a structure and structural damage is monitored by acoustic emission techniques, errors can occur in the analysis of the data using the system due to the assumption that the speed of sound in structures is uniform in all directions and there is a single mode of acoustic propagation though the structure. However, the speed of sound varies with the thickness and type of material through which the sound is propagating. The speed of propagation of acoustic waves will therefore vary as they propagate through an inhomogeneous structure.
It is therefore an object of one aspect of the present invention to provide a system and method to monitor for structural defects in structures on the basis of acoustic emission from such defects.